According to one known technique, aerodynamic measurement probes comprise a part that is able to rotate and is intended to orient itself in line with the airstream surrounding the aircraft on which the support is mounted. The measurement of the local angle of attack of an airstream against the fuselage of an aircraft is an essential parameter for piloting the latter. It makes it possible to define the direction of the velocity vector of the aircraft with respect to the ambient air surrounding it.
The use of a moving vane poses the problem of the sealing of the vane with respect to its support embedded in the fuselage of the aircraft. It is necessary to provide means to prevent or limit the penetration of liquid into the mechanism of the probe. Various solutions comprising sealed rolling bearings, involving dry friction or viscous friction, have been envisaged to resolve this difficulty. These solutions have variable performance levels in terms of sealing and can be dimensioned in accordance with the requirements of the aircraft. However, dry or viscous friction between the pin and the support generates a torque which resists the rotation of the vane. In order to meet the highest demands in terms of the precision and response time of aerodynamic probes, it has been envisaged to employ pins that rotate in a frictionless manner with respect to the support. In order to allow free rotation of the pin, the rotating pin is connected to the support by a set of bearings for guiding in rotation, without seals or other sealing means in contact with the pin. It is thus conceded that liquid can penetrate at the interface between the pin and the support, through the functional clearance necessary for rotation. Thus, a breathable device is referred to, able to accept by design a certain quantity of penetrated liquid or the presence of moisture.
For these devices having a pin that rotates in a frictionless manner, there is nevertheless a desire to limit the quantity of penetrating liquid. An excessive quantity of liquid stagnating in the mechanism is likely to generate premature wear to the mechanism by corrosion. One known solution is to employ mechanical protectors for preventing direct penetration of liquid along the pin.
An aerodynamic probe can be mounted at various locations on the fuselage of the aircraft. In order to measure the sideslip of the aircraft, it is necessary to position the probe near to the vertical axis of the aircraft. In order to measure the angle of attack, it is necessary to position the probe near to the horizontal axis. FIG. 1a describes the fitting of a probe 11 on the fuselage 10 of an aircraft shown in cross section. In this example, the probe is fastened to the lower hemisphere of the fuselage, or in other words below the horizontal axis 12, near to this axis. As shown in a cross-sectional view in FIG. 1b, the probe 11 comprises a shaft 20 that is able to rotate in a support 21 about a longitudinal axis with the reference X, and by means of a set of bearings 22. The shaft 20 comprises a disc 23 separating an outer part 20a of the shaft, located outside the fuselage, from an inner part 20b of the shaft, located inside the fuselage. The support 21 comprises a cover 25 provided with a circular opening 26 through which the shaft 20 passes. The disc 23 and the cover 25, disposed opposite one another in the region of the circular opening 26, limit the penetration of liquid from the outside of the aircraft towards the inner part of the probe. For a probe mounted under the fuselage of the aircraft, as shown in FIG. 1a, liquid that has penetrated can be evacuated through the circular opening under the effect of gravity.
In the case of a spray of liquid directed directly towards the shaft, significant quantities of liquid may penetrate, along the penetration path 28 shown by way of solid lines. For example, the aeronautical standard DO160, which specifies harsh conditions of a spray of pressurized liquid directly onto the pin of the probe, is known. Typically, the probe is exposed to a pressurized jet of 60 000 Pa generated by an outlet nozzle with a diameter of 6.35 mm. In order to meet the most recent requirements of aeronautics, it is thus desirable to improve the efficiency of the evacuation of penetrated liquid by gravity from the probes having a pin that rotates in a frictionless manner.